[Ada] Fix spurious warning on unreferenced internal generic instance

[Pierre-Marie de Rodat via Gcc-patches]

This patch removes a spurious warning, saying that an internal entity of
a generic formal package is unreferenced. The immediate cause of this
warning is that the internal entity is explicitly flagged as coming from
source.

The explicit flagging was added decades ago to fix a missing
cross-reference in the ALI file. Apparently these days the
cross-references work fine without this flag.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_ch12.adb (Analyze_Package_Instantiation): Remove dubious
        call to Set_Comes_From_Source.
diff --git a/gcc/ada/sem_ch12.adb b/gcc/ada/sem_ch12.adb
--- a/gcc/ada/sem_ch12.adb
+++ b/gcc/ada/sem_ch12.adb
@@ -4297,7 +4297,6 @@ package body Sem_Ch12 is
 
       if Nkind (N) = N_Package_Instantiation then
          Act_Decl_Id := New_Copy (Defining_Entity (N));
-         Set_Comes_From_Source (Act_Decl_Id, True);
 
          if Nkind (Defining_Unit_Name (N)) = N_Defining_Program_Unit_Name then
             Act_Decl_Name :=